The present invention relates to carbonless copy paper and coatings therefore containing load bearers and to methods for producing such coatings. More particularly, it relates to a coating slurry containing microencapsulated load bearers having a non-rupturable core material, which are produced in situ so as to be interspersed among rupturable, dye precursor-containing microcapsules and designed in such a way that when the slurry is coated onto the surface of a sheet of carbonless copy paper, a uniform, even distribution of rupturable and non-rupturable microcapsules results which, in turn, promotes a clear, sharp image on the copy paper.
The use of load bearers interspersed with rupturable, dye precursor-containing microcapsules on the CB (coated back) side of carbonless copy paper, or in self-contained carbonless systems, to prevent premature rupture of the dye precursor-containing microcapsules is well known. This technique prevents unwanted smudging and discoloration of the paper due to low pressures applied thereto during storage, transportation, and routine handling. Many attempts have been made to develop a suitable load bearer capable of protecting the dye precursor-containing microcapsules from premature rupture under low pressures yet able to avoid interfering with the production of a clear, sharp image upon the application of direct pressure to the paper substrate, such as from a pen or typewriter key, by not prohibiting the dye-precursor from flowing from the intentionally crushed microcapsule on the CB sheet to the CF (coated front) sheet directly below. The CB sheet is superimposed on top of the CF sheet and the CF sheet is coated thereon with a layer of color-developer which reacts with the dye-precursor to form an image. To the extent that this reaction mechanism is interfered with, the image thus produced will be blurred or broken.
The current approach to the problem of premature rupture is to add inert particles to the microcapsule slurry prior to coating. These particles, which serve as load bearers, are much larger than the microcapsules to give protection thereto from low pressures. Starch balls and cellulose floc are the most common materials chosen. This approach is represented by U.S. Pat. Nos. 3,996,061; 4,280,718; and 4,404,251. Other materials have also been tried. For example, Sandberg in U.S. Pat. No. 2,655,453 teaches the use of glass beads, rounded white silica sand, casein particles, and vinyl acetate polymer as load bearing materials.
Myers et al in U.S. Pat. No. 4,211,437 discloses the use of large agglomerates of kaolin as stilt material. While he refers to his invention as a kaolincontaining "capsule", it is not, in fact, a capsule but rather is an agglomeration of kaolin particles bound together in a very large coacervated mass (see FIG. 2). These agglomerates of kaolin are 2 to 12 times larger than the microcapsules used therewith (col. 2, lines 50-57), have 1/5 to 1/3 the weight of the microcapsules (col. 4, lines 23-26), and are produced in an entirely separate process from that used to produce the microcapsules.
Matsushita et al in U.S. Pat. No. 4,411,451 discloses the use of a wax coating on the CB sheet to improve the transferability of dye-precursor from the CB sheet to the CF sheet (by preventing the dye-precursor from being absorbed onto the CB paper substrate). However, Matsushita does not teach the use of wax for load bearing purposes. Rather, Matsushita states that traditional materials such as starch balls are used for load bearing purposes (col. 3, lines 23-29).
These traditional approaches to the problem of premature rupture have major disadvantages. Differences in density, particle size, and colloid stability between the microcapsules and the load bearers result in their separation or classification during storage, application, and drying. The separation of the microcapsule/load bearer slurry results in uneven coating on the CB sheet. Such uneven coating in turn reduces the clarity or sharpness of the image produced and/or results in a broken image.
In the case of starch, whose density equals 1.4 and particle size is 18 microns, the ratios of its density and particle size to that of the typical microcapsule, whose density equals 0.98 and size is 3 to 6 microns, are 1.4 and 6 to 3, respectively. As a result, on storage the starch particles tend to settle while the capsules remain suspended or float. The slurry must be thoroughly mixed before use, and the stirring maintained throughout the coating operation to ensure a uniform mixture. The large size ratio between the particles also means a strong tendency towards separation during application and drying. This characteristic is generally recognized as the result of velocity differences (different mobilities) among the differently sized particles in the coating currents produced during application and drying. The large particles collect in regions of little flow, and the smaller particles in regions of high flow. A poor coating pattern can easily result. This pattern in turn can reduce the clarity or sharpness of the image produced by the CB.
This separation can be further exacerbated by a second type of separation induced by differences in the flocculation rates between the two types of particles present. Since the microcapsule and the starch ball have different surface characteristics in terms of their chemical nature and polarity, their colloidal stabilities in a given binder solution at a specific viscosity are not identical. A different colloidal stability means different flocculation rates resulting in the formation of larger flocculants of one particle compared to those of the second type of particle. This non-uniformity again produces a poor coating pattern. Void spaces due to starch flocculants can occur which in turn produce a broken image, and an overall deterioration in image quality similar to those mentioned previously.
Unrelated to the problem of premature rupture is U.S. Pat. No. 4,416,966 to Sanders et al. This patent discloses the use of photohardenable compositions contained within rupturable microcapsules. Upon exposure to radiation, those microcapsules thus exposed become hardened and non-rupturable. This feature is used to facilitate the imaging process whereby discrete portions of an imaging sheet containing photohardenable microcapsules are exposed to radiation. The entire sheet is then subjected to a uniform rupturing force so that only the unexposed microcapsules rupture and thereby produces a desired image. U.S. Pat. No. 4,554,235 to Adair et al relates to an improvement to the Sanders et al invention by further producing a high gloss image. Neither of these patents teaches the use of hard microcapsules as load bearers. Rather, the Sanders et al and Adair et al inventions teach the use of hardened microcapsules as part of the imaging process. By the time the Sanders et al and Adair et al microcapsules are hardened, they have already been coated onto the CB sheet and those sheets have already been handled, transported, stored, etc. If a load bearing function were to take place in the Sanders et al or Adair et al inventions, the hard microcapsules would have had to have been present much earlier.
The need thus remains for an improved load bearer having similar size, density, and surface characteristics as the rupturable microcapsules slurried therewith so that a uniform, evenly distributed CB coating can be achieved which in turn promotes a clear, sharp image.